1. Field of the Invention
The invention concerns a procedure and a device for authenticating mobile telephone subscribers of a digital mobile communications system against an authentication entity, and for determining the correct encryption key required for an encoded voice and/or data transmission.
2. Description of the Prior Art
The background of the invention is the capability which is currently being investigated and developed of coupling a mobile telephone network with a cordless telephone system (HBS: Home Base System). Future mobile stations can then be operated not only alone in the mobile telephone network, but also in the home area in conjunction with a cordless telephone system, as a sort of xe2x80x9csuper cordless telephone.xe2x80x9d An important condition for doing this is to find a way that the authentication and voice/data encryption algorithms which are used in the mobile telephone network can also be used in the home area, in conjunction with the cordless system.
For system security, GSM mobile communications systems use several encryption algorithms (A3, A5, and A8) to authenticate subscribers against the network and to encrypt data transmission. To do this, each time a connection is established between the network and a subscriber the authentication center transmits a random number (RAND) over the radio interface to the subscriber""s personal component (SIM card) which both the SIM and the authentication center encrypt to a value SRES using the encryption algorithms with a Ki key which is identical in both of them. The SRES value determined in the SIM is transmitted to the authentication center, and is compared with the SRES value determined there. The subscriber is positively authenticated only if the two independently determined values coincide.
Now, if is desired to apply this authentication procedure to a cordless telephone system, then this would mean that the fixed station of the cordless telephone system would have to implement the necessary encryption algorithm and that it would have to have the secret Ki key stored in it. The mobile telephone network operator would find this untenable for security reasons alone, and it would also involve great technical and administrative expense.
Therefore, this invention is based on the task of specifying a procedure and a device for authenticating mobile telephone subscribers against an authentication entity and for determining the correct encryption key required for an encoded voice and/or data transmission, which is simple and cost-effective to implement and which provides good security.
To solve this task, a procedure having the features of claim 1 and a device having the features of claims 9 and 10 are specified.
An essential advantage of the invention is that the device for determining the authentication and encoding parameters
neither requires a connection with the authentication center of the mobile telephone network (e.g., to the MSC),
nor does it have to contain a special smart card which has, among other things, the authentication algorithm used in the mobile telephone network (for GSM: A3/A8) and secret data (Ki) of the mobile telephone subscriber to be authenticated.
This invention obviates the need to make the authentication algorithm available on the part of the authentication entity of a cordless telephone system and the need to have coordinated keys (Ki) in the cordless telephone system and the SIM.
Another advantage is that the cordless telephone system, especially the fixed station (HBS), requires only slight modifications to be able to perform the functions required of an authentication entity.
This is accomplished by giving the cordless telephone system the ability to execute both the authentication algorithm of the smart card used (SIM: Subscriber Identity Module) and also giving it at least one secret key coordinated with the SIM. If the cordless telephone system meets these two requirements, it is capable of performing an authentication procedure with a logging-on SIM according to the challenge/response procedure which is identical to that which is used in digital mobile telephone networks, especially GSM.